This invention relates to purified metal compositions, to novel metal compositions free of secondary phase material and to a method for making the metal compositions. More particularly, this invention relates to a process for making such compositions wherein a portion of the metal sample is removed as a liquid during heating at elevated temperature and pressure.
Fractional crystallization as a means for purifying metal alloys is well known. The available purification procedures are based upon the fact that when an alloy which solidifies over a temperature range is taken to a temperature within its liquid-solid range, the solid phase has a composition different from the liquid phase. The average composition of both the liquid and solid portions of the metal alloy also depends upon the temperature of the liquid-solid mixture. By controlling the temperature of the system, the impurity to be removed can be isolated in either the solid or liquid phase. Thereafter, the solid and liquid phases are separated by physical means and the pure fraction is recovered.
A wide variety of physical separation means have been proposed which, combined with fractional crystallization, are designed to recover pure metal alloys or metals. For example, centrifugal filtration has been proposed wherein the liquid-solid mixture is subjected to centrifugal forces up to about 2,500 g and the liquid under this force is passed through a filter comprising all or a portion of the wall of the container housing the composition or comprising the solid metal composition itself, as disclosed in U.S. Pat. No. 3,374,089 to Robinson et al, issued Mar. 19, 1968. Alternatively, centrifugal sedimentation has been proposed. In this process, the liquid-solid mixture also is subjected to centrifugal forces to cause the solid phase to accumulate on the interior wall of the container housing the mixture and the liquid phase to accumulate within the central portion of the container. The liquid then is recovered selectively, such as by decantation. Also, the process disclosed in U.S. Pat. No. 3,211,547 to Jarrett et al, issued Oct. 12, 1965 includes a step of forming crystals on the top surface of a liquid-solid aluminum composition allowing the crystals to settle and then compressing up to one-half of the surface area with tampers. The liquid then is recovered. Also, it has been proposed in British Pat. No. 508,925 to heat a metal alloy to form a liquid-solid mixture and then subject the mixture to a pressure of at least 500 atmosphere in order to force the liquid through a filter to leave a solid.
While these processes are capable of increasing the purity of metal compositions, they are undesirable in that a substantial portion of the liquid remains unseparated from the solid and the "efficiency" of the process is low, i.e., that often the separation process must be repeated a number of times to obtain the desired purification. The process provided herein overcomes these objections and provides efficient, economical means of purifying metal.
All cast alloys posses significant dendritic microsegregation after solidification. The secondary phase that results from "non-equilibrium" solidification limits the alloys that can be practically cast and worked, and the properties of alloys that are cast. For example, significant increases in strength of 7000 series aluminum alloys could be achieved by increasing the alloy content above that of 7075 alloy or its newer modifications. However, when such increases are made in ingots, the amount of eutectic increases to a point that ingot-working is difficult or impossible, ingot cracking is a problem, and the eutectic is not fully solutionized with practically obtainable cycles. Thus, methods such as atomization and thin plate casting have been employed to obtain a fine enough particle size so that solutionization can be obtained before extensive working. Secondary phases are well known to limit the mechanical properties of ferrous and superalloys, examples being the sulfides in steel and the alloy eutectic in .gamma.' strengthened superalloys. If these alloys could be produced with a structure exactly like that now achieved, but without precipitation of this secondary phase at the end of solidification, significant increases in room temperature and elevated temperature properties could be obtained.